It doesn't though, at the end of the day there's an objective amount of CI and an objective amount of CO. Further, CO isn't just "activities", for instance you burn calories merely by existing. Things you're describing will impact CI and CO, but at the end of the day if one had the ability to fully and 100% accurately measure CI & CO it'd be apparent that the math works.
But this is why "it's just CICO" is at best a tricky phrase. Because the hard part is in the nuance you describe.
Math truth is not always good policy guidance.
It is true that CI==CO. It must. It is not true that telling someone that CI==CO is a good way to get them to manage their weight, because (as mentioned) it's hard to measure and (as I added) even if you measure correctly, you _reduce likelihood of compliance_ by ignoring appetite effects when you call all calories equal.
I think we agree, just trying to find the right words anyway.
A 5K run for 30m burns ~300-600 calories. A single serving of a candy bar is ~250 calories. You NEED to restrict the CI portion since its the easiest part of the equation to control.
The typical person who is maligning CICO because it doesn't explain why they're still fat, that person would do much better by better focusing on CICO and performing the all time best exercise for weight loss - the "table push away"
Meanwhile the typical strict advocate of CICO as the end all be all also needs to understand that there's a lot of nuance in both the CI and CO variables.
Not the GP, but based on their abstinence-only sex comment I imagine the point being raised is that it's something that is technically true but not a practical guideline.
CICO is a true statement. But you're not going to be able to accurately measure CI and especially not CO. So why bother using that as your guideline on how to proceed? Instead it is known and understood that there are mechanisms for things such as improving CO efficiency, and it's much more practical to focus on that.